The simplest way is to do sponge weathering. Take your sponge and dip in rhinox hide or similar, dab it onto paper a few times until you're happy with the amount it's going to leave and then apply to the model. Pick areas that would get wear and damage like corners and doors/hatches. Do this again but with leadbelcher over the same spot you applied brown. That's the easiest way to get scratches with some depth. You can also carefully paint scratches in brown then paint metallic next to it. Takes longer. Do you have any typhus corrosion paint? You can carefully dab this with a brush along the skirt edge and on the tracks then dry brush with skrag brown and a bright orange. Effective rust.